Gas turbine engines, such as turbofan gas turbine engines, may be used to power various types of vehicles and systems, such as aircraft. Typically, these engines include turbines that rotate at a high speed when blades (or airfoils) extending therefrom are impinged by high-energy compressed air. Consequently, the blades are subjected to high heat and stress loadings which, over time, may reduce their structural integrity.
To improve blade structural integrity, an internal cooling system is, in some cases, used to maintain the blade temperatures within acceptable limits. The internal cooling system directs cooling air through an internal cooling circuit formed in the blade. The internal cooling circuit consists of a series of connected, serpentine cooling passages, which incorporate pin fins, turbulators, turning vanes, and other structures therein. The serpentine cooling passages increase the cooling effectiveness by extending the length of the air flow path. In this regard, the blade may have multiple internal walls that form intricate passages through which the cooling air flows to feed the serpentine cooling passages. To further minimize blade temperatures, the blade typically includes a tip recess across its top wall. The tip recess may also be configured to minimize flow leakage across the blade top wall.
To form the above-mentioned cooling features in the blade, an investment casting process is typically employed. In one example, a single ceramic core including a bottom core portion and a top core portion is used. The bottom core portion is shaped to complement the internal cooling circuit, and the top core portion is shaped to complement the tip recess. The ceramic core is disposed in a ceramic mold having an inner surface shaped to complement an outer surface of the blade. The two ceramic core portions are held spaced apart from one another by ceramic core bridges or quartz rods to form one integrated core. Molten metal is then injected into the ceramic mold around the ceramic core. After the metal solidifies, the ceramic is leeched away from the metal, thereby exposing the blade and tip wall holes formed by the ceramic core bridges or quartz rods. The holes are utilized to flow cooling air or are plugged with a braze material to prevent cooling air leakage. In another example, a core is first used to form the blade, and the tip recess is then subsequently machined into the blade.
As engine operation temperatures have increased and internal cooling circuit designs have become more complex, some drawbacks to the above-described blades have arisen. Specifically with regard to those blades having tip wall holes, the braze material in the holes may melt when the blades are exposed to higher temperatures. Consequently, the blade may not cool as intended when air leaks out of the holes. As for blades having machined tip recesses, the core may shift out of place within the ceramic mold at some time during the manufacturing process. As a result, the tip wall may be misshapen and the tip recess may be imprecisely formed. To prevent this, costly precision locating strategies, such as repeated x-ray verification techniques could be employed; however these techniques would also increase blade manufacturing costs.
Hence, there is a need for an improved method of making a blade having a cooling system that is capable of cooling a blade tip in extreme heat environments. It would be desirable for the method to be cost-effective and relatively simple to employ.